1. Field of the Invention
The invention relates to a polarizable electrode for use with a device such as an electric double layer capacitor and a cell.
2. Description of the Related Art
An electric double layer capacitor (EDLC) has been widely used as memory back-up devices. In such devices, the electric double layer capacitor is electrically connected in parallel with a cell or a DC battery converted from a commercially available AC battery, and carries out back-up for the devices by electric charges stored in the capacitor when the cell or battery is instantaneously cut off.
A large-capacitance electric double layer capacitor having a capacitance larger than 10 farads has been recently developed. For instance, there is reported an electric double layer capacitor having 500 farads capacitance at 2.3 V working voltage in Proceeding of the 41st Electronic Components and Technology Conference, 1991, pages 531-536. For another example, there is reported an electric double layer capacitor having 1000 farads capacitance at 5.5 V working voltage in Extended Abstract of the 180th Electrochemical Society Fall Meeting, Phoenix Ariz., Oct. 13-17, 1991, Vol. 91-2, No. 80, page 121. These electric double layer capacitors are expected to be applied to pulse power accumulator for use, for instance, with an electric automobile by making the best use of properties thereof that such capacitors have a large-capacitance and a low equivalent series resistance (ESR).
There are used several solid activated carbons as a polarizable electrode for use with the above mentioned large-capacitance electric double layer capacitor. For instance, Japanese Patent Publication No. 4-44407 has suggested a polarizable electrode manufactured by carbonizing and activating phenol resin, polyvinyl alcohol, starch and phenol resin foam obtained from formalin and acid catalyst. Japanese Unexamined Patent Public Disclosure No. 63-226019 has suggested a polarizable electrode manufactured by thermally treating a mixture of activated carbon fiber or powder and resol type phenol resin. Japanese Unexamined Patent Public Disclosure No. 4-288361 has disclosed solid activated carbon manufactured by thermally treating a mixture of activated carbon powder and phenol resin powder.
A large-capacitance electric double layer capacitor is required to be able to supply a lot of current in a moment in order to work as a pulse power accumulator. The polarizable electrode suggested in Japanese Unexamined Patent Public Disclosure No. 4-288361 has micropores including those of activated carbon powder having a radius in the range ten to tens of angstroms, and those which are formed when phenol resin is carbonized and have a radius in the range from 1000 to 2000 angstroms. in such a polarizable electrode having a micro-porous structure, movement of ions in micropores is suppressed. Thus, the above mentioned polarizable electrode poses a problem that if a lot of current is to be discharged, a capacitance thereof is apparently decreased, and hence the sufficient performance cannot be obtained, as reported in NEC Technical Disclosure, Vol. 46, No. 10, 1993, FIG. 10. It is considered that the polarizable electrode suggested in Japanese Unexamined Patent Public Disclosure No. 63-229019 has a similar problem. Thus, it is required to realize a micro-porous structure in which ions can move more easily.
In addition, a maximum current passing through a unit volume of a polarizable electrode is in proportion to a capacitance per a unit volume of the electrode. Thus, it is preferable for a polarizable electrode to have a larger capacitance per a unit volume.
Japanese Unexamined Patent Public Disclosure No. 4-44407 has also suggested a method for manufacturing a polarizable electrode having micropores having a diameter of a few micrometers. In the method, after phenol resin foam has been molded, the molded foam is carbonized and activated. However, a porous electrode obtained by the method has a tow density and a quite small capacitance per a unit volume. In addition, the method has complex steps, and hence the method is considered to need high cost. Furthermore, since the resin is foam is to be carbonized and activated in the method, there can be obtained only a small yield, and further it is quite difficult to uniformly process the resin foam. In addition, the method uses natural products such as starch as a raw material. Such natural products contain impurities such as Na and K, and hence are not suitable for a raw material for manufacturing a polarizable electrode.
The methods for manufacturing powder or fibrous raw material as suggested in Japanese Unexamined Patent Public Disclosures Nos. 63-226019 and 4-288361 pose a problem that powder scatters during manufacturing a polarizable electrode.